The present invention relates to wave soldering of elements such as printed wiring boards, and more specifically the present invention relates to wave soldering in an atmosphere which substantially excludes oxygen.
For many years wave soldering systems have required flux to promote wetting and good solder joints through removal of surface oxides on the metallic surfaces to be soldered, both on the circuit boards and on the component leads of printed wiring boards. More recently inert gas soldering, generally nitrogen gas and/or other inert gases, has shown some benefits in the reduction of the amount of flux required in soldering. The inert gases cover the liquid solder surfaces and prevent oxidation of the solder. By eliminating flux and still being able to prevent oxidation of the solder one avoids the necessity for cleaning after the soldering step and this is normally needed to remove the flux residues. Such an operation is expensive.
As well as inert gas, soldering has been conducted in an atmosphere which substantially excludes oxygen and this is the key to fluxless soldering. Forming gas which is a mixture of nitrogen and hydrogen has certain advantages for soldering in that it is purported to promote hole filling in circuit boards without the use of flux.
Soldering in a shield gas atmosphere substantially excluding oxygen is known. The gas permits fluxless soldering, for instance German patent publication Ser. No. DE 3309648 to Siemens shows the application of shield gas to a printed circuit board in a passage prior to soldering. German petty patent G 85 20 254.1 discloses soldering inside chambers which are filled with shield gas, the items to be soldered are transported on conveyors through these chambers. Vestibules at each end are shown to ensure that a minimum of shield gas escapes from the soldering chamber. German patent application DE 3737563 discloses another system wherein the elements pass in a tunnel-like housing. The housing has a number of curves in it so that the entry and the exit are below where the soldering occurs to ensure that shield gas remains within the tunnel.
Some of the features of shield gas soldering, often referred to as inert gas soldering, are disclosed in an article entitled Soft Soldering Under Cover Gases by H. J. Hartman. The system discloses reflow soldering as well as wave soldering.
One of the dificulties with shield gas soldering systems in fluxless soldering includes the problems of soldering circuit boards and the like which contain plated metallized holes, either with or without component leads which have to be filled with solder. The holes have metallized plating connecting the lower face of the circuit board with internal layers and/or a top layer. Without flux, it is found that with the preferred gas, nitrogen, the soldering operation does not promote the capillary rise or wetting of the solder from the solder wave to wick up the metallized plated hole, and this is an essential specifiction in the entire Electronics industry. Therefore at the present time additional conventional or liquid type low solids fluxes are required to ensure that the solder wicks up the metallized plate hole. By the addition of fluxes one again causes deposits to form on the soldered item, carries or pallets, if used, and on the liquid surface of a solder pot. This may require removing the flux deposit from the soldered item, and carries or pallets, if used. This may cause a layer to form on the solder pot which must be skimmed from time to time.